High-frequency induction tube welding



Alli. 1954 'r. J. CRAWFORD HIGH-FREQUENCY INDUCT ION TUBE WELDING 4Sheets-$heet l Original Filed Jan. 11, 1951 1954 T. J. CRAWFORD2,637,465

HIGH-FREQUENCY INDUCTION TUBE WELDING Original Filed Jan. 11, 1951 4Sheets-Sheet 3 AT TORNE 3K5- g- 1954 T. J. CRAWFORD 2,687,455

HIGH-FREQUENCY moucmu TUBE WELDING Original Filed Jan. 11, 1951 4Sheets-Sheet 4 WELD! N 6 Q0! ROLLS POI NT OF COI L.

SEAM Gun INVENTOR. 790/145 J. C/MWFORD ATTORJVE Y5.

Patented Aug. 24, 1954 HIGH-FREQUEN CY INDUCTION TUBE WELDING Thomas J.Crawford, Berkley, Mich.

nuary 11, 1951, Serial No.

Original application J a 205,511. Divided and this application March 6,1953, Serial No. 340,837

Claims. (01. 2196) method of welding.

Many attempts have been made to produce metal posed seam edges togetherby application of electrical energy thereto. The well-known Yoderelectric resistance-weld tube mills which are now seam. The resistanceto passage of the electric current across such seam serves to melt theabutting edges for an instant, rendering the pressure fully welded byresistance methods.

Attention has been given to the possibility of heating the opposed seamedges to welding temperature by means of a current induced in the tubeblank itself as taught, for example, in Adams Patents Nos. 2,181,445 and2,335,894, a relatively high voltage, low frequency current beinginduced to cause arcing between the spaced seam edges to raise the sameto welding temperature prior to passage between appropriate squeezerolls. It has also been suggested to utilize a high frequency inducedcurrent, as in Leonard patent, 2,205,424, with such current beinglocalized and concentrated at a point within the confines of aninduction coil encircling the tube blank and with the seam edges beingforced together into welding engagement at such point. Due tooperational difiiculties, neither of these last-mentioned methods hasever enjoyed substantal commercial use.

It is accordingly a principal object of my invention 0 provide aninduction tube welder and to weld in a satisfactory manner to producecontinuous lengths of tubing.

Another object is to provide welding apparatus which will accommodateStill another object is to provide means for welding very thin-walledtubing which may, for example, have a wall thickness of about .012 inchin the case of a one inch 0. D. tube.

Other objects of my invention will appear as the description proceeds.

To the accomplishment of the invention may be employed.

In said annexed drawing:

Fig. 1 is a diagrammatic elevational view of a tube mill adapted to beutilized in accordance Fig. 2 is an end elevational view of a preferredform of seam guide comprising an element of my new welder;

Fig. 3 is a side elevational view of the induction coil and associatedsqueeze roll unit which I prefer to employ in conjunction therewith;

Fig. 4 is an end elevational view of such squeeze roll unit;

Fig. 5 is an elevational view of the other end of such squeeze rollunit;

Fig. 6 is a side elevational view of one preferred form of inductioncoil partly broken away better to show the internal arrangement of thesame;

Fig. '7 is an end elevational view of such coil;

Fig. 8 is a diagrammatic top plan view of a portion of the continuouslytraveling tube blank, explanatory of the manner in which the weldingoperation is performed;

Fig. 9 is a diagrammatic representation of the means for supplying thehigh frequency current to the induction coil, the units comprising suchmeans being themselves commercially available;

Fig. 10 illustrates means for introducing inert or reducing gases and/or fiuxing agents adjacent the point of welding;

Fig. 11 is a side elevational view of one of the baffles comprising anelement of such means; and

Fig. 12 is a vertical sectional view taken on the line l2-i2 on Fig. 10.

Referring now more particularly to Fig. 1 of the drawing, the flat metalstrip S is withdrawn from the coil box or uncoiler i and passes throughan edge scarfing device 2 to the power driven roll stands 3 of theforming m'll l, such rolls being operative to advance such strip whileprogressively forming the same to desired tubular cross-section. Thetube blank thus preformed now advances to the welder 5 which comprises aseam guide unit 6, a high frequency induction coil l, and a squeeze rollunit 8. The welded tube next passes through the water cooling section 9,sizing and straightening unit it, and thence to flying cut-off H andrun-out table 12 where the lengths of finished tubing are collected andbundled. An external bead or flash trimmer (not shown) may also belocated intermediate the squeeze roll unit and the cooling section. Allof the units above referred to and which together comprise the completemill are of well-known construction and commercially available with theexception of the welder 5 comprising such seam guide if, induction means1, and the squeeze rolls 8.

The seam guide comprises a stand it carried by a slide Hi mounted onways If for adjustment transversely of the path of travel of the tubeblank B, such adjustment being necessary to over-correct thecircumferential position of the seam if the latter has rotated out ofproper alignment. An adjusting screw is is provided to position suchslide on the ways. A roll ll is carried by a spindle 18 supported inbearing [9 in stand [3 and is provided with a circumferential groove 28adapted to conform to the contour of the tube blank to be welded and tosupport the latter. The seam guide proper comprises two flat metalblades or fins 22 and 23 separated by a layer of insulating material 2!and preferably both supported in an insulated mounting by an outrigger0r bracket 24 mounted on stand l3 for vertical adjustment above roll l1.By insulating the wear resisting blades 22 and 23 from each other, andalso preferably from their mounting, the passage of current across theseam at this point is prevented, it being appreciated that such seamguide will ordinarily be located quite close to the induction unit. Theeffect of such guide is to position and space apart the opposed seamedges 25 and 26 of the tube blank B immediately prior to passage of suchblank through the induction coil, thereby properly positioning such seamfor conjoint action of the squeeze rolls and also, in cooperation withsuch rolls, determining the angle of approach of the opposed seam edgesat the point of welding. Except for the special blades comprising theseam guide proper, the entire unit is of conventional construction. Itmay be dispensed with entirely under certain circumstances where thetube forming mechanism is properly arranged and adjusted to form atubular blank with the seam properly positioned and the opposed seamedges properly spaced. The exact position of the seam is of moreimportance when adjacent squeeze rolls are arranged to engage the tubingclosely to each side of the welded seam than when a ceramic squeezeroll, for example, is mounted directly to overlie such seam. When theseam guide is utilized, it will be of slightly greater thickness thanthe width of the seam coming from the forming mill so as to be effectiveto spread such seam somewhat. The sides of the seam guide may alsodesirably be slightly tapered toward the point of welding, the seamedges usually approaching each other at a fairly un form angle.

The squeeze roll unit which is located immediately subsequent to theinduction coil 1 comprises a slide 21 mounted in ways 28 for adjustmenttoward and away from coil 1 along the path of travel of the work (seeFigs. 3-5 inclusive). Carried by such slide are two parallel upstandingplates 2% and 30 braced at their upper ends by bars 3i and 32interconnecting the same. Such plates are provided with aligned,generally circular central apertures 33 and 34 through which the weldedtube T is adapted to pass. Three arms 35, 36 and 31 are pivotallymounted in equidistant radial slots 38, 39 and at extending outwardlyfrom the periphery of aperture 34 in plate or stand 3%. In theembodiment illustrated, one of such slots extends vertically downwardlyfrom such aperture.

Such arms extend toward coil 1 generally parallel to the path of travelof the tube and pass through corresponding radial slots M, 32 and 43 inthe periphery of aperture 33. Circumferentially grooved squeeze rolls M,45 and 46, preferably of non-magnetic material such as bronze, arecarried in removable mounts 41, 48 and 49 on the ends of such respectivebars adapted to engage and bear against substantially the entireperiphery of the tube being welded. The precise transverse contour ofthe grooves in such rolls will, of course, depend upon the size andshape of the tube being welded.

Screws 58, 5i and 52 are mounted in such plate or strand 29 adaptedadjustably to bear against the corresponding bars or arms 35, 36 and 31to force the squeeze rolls carried thereby radially inwardly against thework. It will be noted that in the arrangement illustrated the weldedseam, being uppermost, is adaptedto pass between the slightly spacedsqueeze rolls ift and 15, and any external bead may thereafter betrimmed away by an appropriate cutting tool in well-known manner. Whenhowever, the formation of an internal bead is not objectionable in viewof the use to which the tubing is intended to be put, the roll carryingstand may be inverted so that the vertically disposed roll 36 will nowbe uppermost and will bear against the tubing in the region of the seam,forcing inwardly any bead which may be formed and obviating anysubsequent trimming operation. In this event,

my purpose, and for operations where welding from V inch to 2 inch 0. D.tubing having a wall thickness of as much as 1 inch such oscillatorhaving a rated output of 20 kilowatts 4 rolls, for example, beingsuitable under some 5 at 400 kilocycles is suitable. An oscillatorhavcircumstances. ing an output of as much as 50 or 100, or even Theinduction coil '1 will preferably be formed more, kilowatts may in somecases be employed. of electrolytic copper tubing in order that a Therectifier may be air-cooled and the oscillator cooling medium such asWater may be ciiculated Water-coiled, and they ale commerciallyavailatherethrough When very high frequencies are ble with such coolingsystems The commercially to be utilized it will be advantageous heavilyto available Allis-Chalmers rectifier may desirably silver plate theouter surface of the coil. As best be slightly modified by providing thesame with shown in Fig. 6, the turns of such coil may be of an outputfilter to minimize the ripple comrectangular cross-section for maximumeirlponent in the output lt d thereby ciency, and where very small sizesof tubing are 5 respondingly minimize the modulation of the to be weldedI prefer to utilize an additional inhigh frequency current so thatperiodic fluctuaternal inductor element 53 rather than to protion of thewelding heat which may otherwise vide a coil of extremely smalldiameter. As best be apparent When Welding at high speeds (60 shown inFig. 6, such element may comprise a feet per minute, or greater) issubstantially generally cylindrical copper shell 54 fitting closeeliminated. Such rectifier may also be modified ly within 0011 l butinsulated therefrom Such y ub tut of y atwn tubes (ionicconducinsulation may conveniently be in the form of tion tubes with gridcontrol) for some of the tape wound upon such shell Alternat vely, atwo-element mercury vapor tubes usually pioslight air gap may beprovided between the shell vlde h r y f ll e i m nt of the and 0011,particularly when the same are lndewelding heat Alternatively, asaturable reactor pendently supported At the end towaid the may beincluded in the power lead to the rectifier squeeze roll unit, suchouter wall tapers inwai dly fer e Same D D It y additionally be ingeneral frusto-conical conformation [in indesirable to incorporate meansfor Compensat ner conical tapering wall 55 is brazed or silve forfluctuations in the line Voltage. soldered thereto forming a closed,generally sun This Allis-Chalmers Electronic Heater vacnular chamber553. Such chamber is, however, uum tube oscillator has the furtheradvantage divided along a lower median line by spaced parfor my purposeof incorporating provision for titions 51 and 58, which may be brazed inplace, inductive coupling of substantially all of the leaving a narrowlongitudinally extending radial oscillating tank circuit inductance intothe outgap th b t inner and Outer n 5d put or work coil circuit; the lowpower factor of and 55 likew se be ng he e in e rupteo so tha the workC011 Oi inductor together With its load the cooling medium such as waterntroduc d makes this type of couple highly desirable to through one ofconduits 59 and en 11 be caused obtain eiiciency of operation when thework is of to circulate through such chamber and escape on-ferrous ornon-magnetic material. The through t other such condum 'The n 1116111.40 entire tank inductance coil is used as the primary bars 54 and 55thus form a single turn coil or 0 1. concentrator within the inductorcoil I A The most de ble frequency to be employed Micarta or likeinsulating material plug bi is W l oi Cou se, vary somewhat depend pfitted in the wider flaring end or such concen the p l l ulai operationcontemplated, it being t t r nd provided with a e ntlal aperture 52necessary to consioei the diameter of the tube, therethiough for passageof the reformed tube the permeability and the electrical resistance ofblank B closely following seam guide unit the metal of which the tube isformed the weld- Such Micarta plug serves itself as a guide for the instemperature or the particular metal the tube blank and maintains thelatter out of confic h at of the metal and the rate at which tact withthe concentrator. This plug, and theret e ding operation is to beperformed In by the concentrator, may be supported by a suit- 01 ryoperations, I have found 9600 cycles to able bracket eitherindependently of the coil or be a piactical effective minimum, althoughin supplementing the support afforded by such coil eXeeptlOIlal easesWhen W d g very l rge diamand interposed insulating tape. The frustoeterthick-Walled tubing a f equ ncy as low as conical portion 63 ofconcentrator 53 facilitates 55 3090 cycles may on Occasion be emp oyed.While generation of a high frequency induced current the heat input y,of Course, be Varied y edin the tube blank in a region closely adjacentl g the initial generator voltage (1 e that of th following queeze 11the power source), the operation will be inefii- Inductor coil 1 isenergized by power from line cienb if the frequency mp y d is rossly 54which is first; rectified by conventional m different from thatdetermined tobe desirable as fier and the desired high frequencygenerated a result of the above COIlSidemtiOns. by a conventionalgenerator such as oscillator The concentrator 53 Operates bo h as thesec- 66 The 0011 leads may be clamped in the ends ondary of atransformer (inasmuch as it lies of appropriate bus bars, withconventional t within coil 7) and also as an inductor relative coolingmeans provided for both such bars and 65 to the tube blank B The portion2' of inner wall 0011 Various types of high frequency generators 55 ofsuch concentrator 53 which extends parallel are commeicially availablesuch as the Westingto and closely adiacent the tube blank may be house,G. E and Reliance Electric rotating gensubstantially shoiter than theaxial extent of erators, the A ax Electrotheimic o1 Coirugated coil 7,thei eby concentrating the current in this Quench Gap Co spark gap typegenerator, the 70 region, as well as protruding from the coiloscillating arc type generator and the Alli toward the squeeze rollunit, so that the region Chalmers, Westinghouse and G. E vacuum tube inwhich the induced current is concentrated may oscillators. I have foundthe Allis-Chalm rs lie closely adjacent such squeeze rolls while at thevacuum tube oscillator sold under the trade name same time facilitatingthe mechanical installa- Electronic Heater particularly satisfactory for76 tion of the inductor unit in that the leads to coil l may extend fromthe latter without interference with the squeeze rolls. The concentratormay desirably be silver plated similarly to coil 1 when very highfrequencies are to be employed. The flux induced by outer coil 3 linksthe outer portion of concentrator shell 54, inducing a volt age aroundthe exterior of such shell and across the open gap between partit'ons5'! and 58; the current thus produced flows for any half cycle inwardlyalong either partition 51 or 58 and around the small diameter innercylindrical shell portion 61, returning to the outer periphery of shell54- along the other of such partitions iii and 58. Since high frequencycurrents tend to circulate along the shortest path most of the currentproduced in the full length of outer shell 54 is concentrated in theshort inner portion 61, thereby setting up the desired intense field,through which the tube blank passes, in the proper location, i. e.,ofiset from coil 1 toward the squeeze rolls. In view of such portion 67being thus axially ofi'set, substantially beyond the confines of coil'1, very little intreference results between their respective fields ofopposite polarity, leaving the field of portion 61 fully eiiective toperform its work.

When welding certain metals and alloys such as, for example, Everdur (asilicon bronze alloy), there may be a clusions in the weld, and it isimportant either to prevent the formation of such oxide or to flux thesame .away during the welding operation. I accordingly also providemeans for the introduction of an inert gas such as helium to the weldingregion to prevent oxide formation or for introducing natural gases orthe like having entrained therein a solution of methyl borate inacetone, this latter serving as an excellent flux particularly efiectivewhen welding steel, copper alloy tubing and the like. ihe devicecomprises a length of copper tube 58 adapted to enter the preformed tubeblank B immediately following the last roll of forming mill l whichbears against the inner surface of such blank. It is important that atleast the portion of tube 83 passing through the inductor field be ofnonmagnetic material and of quite small diameter so as not to rob thefield. Copper tubing be then extends generally axially of the tube blankto a point somewhat beyond the squeeze rolls of squeeze roll unit 8,this end of tubing .68 being plugged. A Micar-ta or similar wear andheat resistant disc 6% is mounted on tubing '58 adjacent suoh latter.end to serve as a baflie, being of only slightly less diameter than theinner diameter oi the welded tube T. A plurality of orifices areprovided tubing cc adjacent such baiiie so that a gas may be introducedtherethrough to the interior of welded tube T somewhat beyond the pointof welding of the same. Such gas will then escape therefrom through theseam between the opposed seam edges 25 and 2G in the region where suchseam edges are heated through the agency of inductance coil 1. Anothercircular baffle H, likewise of insulating material, will preferably bemounted on tubing 68 at a point intermediate the seam guide 2! and coil1 thus to direct the escaping gas upwardly through the open seam. Asshown in Fig. 11, the battles G9 and it will preferably be provided withperipheral grooves such as 22 and ridges E3 to obtain a better seal inwell-known manner. It thus becomes a simple matter to introduce a gaswhich may be inert, such as helium, of a retendency to form oxideinducing nature such as hydrogen or natural gas, or have entrained anorganic flux in vapor form.

It will, of course, be understood that the particular gaseous mediumutilized may vary depending upon the particular welding operation, andin many instances the entire device for its introduction may bedispensed with. When such device is employed, however, it may alsoconstitute a convenient supporting means for an inner burr or beadtrimming tool i l mounted on a conventi-onal carriage or mandrel Msecured to the end of tube 58 and adapted to remove any bead which maybe formed on the inside of the tubing along the welded seam. The use ofinternal bead trimming tools is, of course, broadly old in the art.Devices for entraining a fluxing material in an appropriate gaseousvehicle and known as gas fluxers are well known in the art andcommercially available, being employed at the present time to assistbrazing and torch welding operations.

For a fuller appreciation of the principles of my invention and thetheory underlying the same, reference may now be had to Fig. 8 of thedrawing as explained below. ihe open seam of the preformed tube blank Bis engaged by the insulated seam guide H, 22, 23, both to space edges 25and 25 slightly farther apart and also properly to register the blankcircumferentially relative to the squeeze rolls. Such seam guide isrelatively thick as compared to similar guides proposed in prior artattempts and is also relatively close to the squeeze roll unit (althoughthe inductance coil is of course interposed therebetween) so that theopposed seam edges are too widely spaced apart to permit any appreciablearcing thereacross under conditions of use and approach each other at anangle which is not as acute as in prior art attempts. Consequently, thecurrent produced by the voltage induced circumferentially of tube blank38 within the confines of the induster must of necessity travellongitudinally of the blank to pass across the seam at or beyond thepoint of welding and beyond the confines of such inductor (see Fig. 8).The fact that the seam edges approach each other at a relatively largeangle not only discourages arcing but also renders it a simple matter tomaintain such point or juncture uniform in relation to the inductor andsqueeze roll units. Where the seam edges have in the past been caused toapproach each other at a very acute angle, there has been a decided.tendency for the point of welding juncture to shift back and forth withthe result that sometimes holes are burned in the welded seam and atother times only intermittent stitching is accomplished.

As above indicated, the blank B passes from the seam guide unit (whensuch is employed) immediately to the inductor unit, traveling axiallythrough coil l and the concentrator 53 if the latter be employed. Suchconcentrator will ordinarily only be util sed when welding the smallerdiameter tubes as, for example, one-half inch Q. 1)., or less. Portioniii of the concentrator will he closely adjacent but out of contact withthe tube blank passing therethrough. When no such concentrator isemployed the coil 1, ordinarily one or more turns of water-cooled inchcopper tubing, will similarly closely encircle the tube blank B. Thenumber of turns of the coil will be selected to obtain the maximum poweroutput from the particular oscillator employed. The path followed by thehigh frequency current proindicated in Fig. 8, the generallycircumferential jump the open seam it crosses over at the point portionthereof lying substantially within the of juncture of the seam edges.The squeeze confines of coil 1 (or of concentrator neck 61, rol s a e ajus d o t Such point of juncture when employed). 5 is slightly inadvance of the center line of the In selecting the frequency of thecurrent to squeeze rolls and consequently the interfused be passedthrough such coil, Weight will be given ed f th seam are h r p nslightly urged to the following considerations. The induced together ina u equ n f r cpe i nby voltage will be proportional to the frequencyand action of the T0115. i fo g Operatlon to the cross-sectional area,defined by t t affords improved grain structure in the weld and and thislatter is proportional to the square of the ensures a perfect joint Withno p diameter whereas the resistance of the tube is The point ofbringing together F PlP directly proportional t t diameter, Conse seamedges relativeto the inductor is critical to duced in the tube blank isdiagrammatically cumferentially of the blank but being unable to willdecrease with increase in the tube diameter. be Spaced axially Om theFonfines of 131.19 efiec- In order t bt i t necessary heat input tivecurrent-inducing portion of such inductor (12R) it will Obviously benecessary to employ a so that the current induced n the tube blank W111higher frequency the higher the resistance of the be caused flowlengthwlse of such blank to particular metalbejng Welded pass around theopen end of the seam. The

Likewise, when Welding a metal having a h gh Skin effect and i efiect of511.011 mwelding temperature, a relatively high frequency duced e tonsequen ly achieve my Object of must be employ-ed to raise the seamedges to such concentratmg Weldmg Current (and temperature heatdeveloped thereby) at an extremely localized Similarly, the greater thespecific heat of the m (the point of mncture of the seam edges) metalbeing welded, t higher the frequency Without greatly heating the body ofthe tube. which m t be employed It will of course, be Thus, if the seamedges werebrought into contact appreciated that the foregoingexplanation before passage through the inductor there would assumes thatother factors are maintained conbe shght resistance heatmg at Such.edges no stant since the rate of travel of the tube blank m concemratlonof current denslty at Smgle within the confines 01 the C011, the heatingeffect i of the vanables referred to From a would b somewhat enhancedbut there would be ica operating oint or view however, such speed a Vely1a we power loss and the D1 ess W 0111 d not 0 travel must be maintainedat a mechanically bfl comnigciapy feasible By bringing the andcommercially feasible rate. V

posed seam edges together sufficiently beyond a t? emp 10y ed m practlcemay Ordl" the inductor the induced current is caused to many range 10mabout 9500 to 400,000 cycles concentrate at such edges and particularlyat n may exceed 500,000 cycles for Very small the point of contact orthe latter. The most diameter tubes, 10,000,000 cycles theoreticallybeing feasible. In the case of very lar e diami eter tubes the frequencyy be as as 2800 40 applied without unduly heating the body of theOccurrence beyond the exact point of Welding Since high T V ta nfrequency currents do not penetrate much below 8 0 agv eppealmg aclossthe Seam edges the surlace of the metal conductor and tend to ahead ofthe point of welding will be insuflicient follow the path of leastimpedance (the vector gg g i g i zg ggi tarcmg getween the sum ofresistance and reactance) 1t W111 readily broh t t Jus as sue edges arebe seen that the culrent induced in the tube g Oge er fapomt of Weldmgsputter blank W111 pass on cumferentially of tne lnner and 1 may takeplace Whlch has the effect of Clean outer sur aces of the latter andthen along the ing the opposed edge surfaces The voltage selected will,of course, depend on the material to gi gfjg gg ifi g v i fi g fii ggg225g be welded and speed of operation. For reasons This skin effect ismuch enhanced in the region 5 F i fi under usual operatmg 69mm of theseam edges by the proximity eifect relons, 14 e v0 age is raised to atoo high level l the result will be to burn out the metal. sulting fromthe opposite dnections of flow of Despite the proximity of the inductorunit to zgg f gi g ggfi z 22:51 gg g e $55255; E s: Ztiitfitiit?$51-33;Zitltititfi?$551325 in extremely high density being achieved.field of such inductor. It is nevertheles" gens erally desirable toemploy non-magnetic matei 1t g gf f t- 1 11 t fi siieii fi rials for therolls, and where a roll is used dif g 35 53 1 1 d 95 l rectly over theWelded seam it should preferably g u y g y a l a be of insulating andrefractory material such as igg ggzg gfir sggt digit? s e ff e fil hi lbonded Sapphire Vitreous aluminasapphire heat such edges. Just as theedges are about to rolls are Sultable but rather expenslve Due toucheach other the low voltage employed may the fact that my new rocess actsto concentrate be s fficient to cause sputtering and this has theWelding heat at the point of Welding to a the advantage of cleaning thejuxtaposed survery high degree it is not generally necessary to faces asin a flash Weld, particularly advaneheit such heavy p essu es on tnesqueeze rolls tageous in the case of aluminum. The seam edges are notraised to welding temperature of crushing the same.

As indicated in Fig. 8, the current induced in juncture. the tube blankby the inductance coil flows cir- The increase in current density atsuch point of juncture is so abrupt that a substantial increase involtage (other welding conditions remaining normal) will not causearcing to develop back along the open seam but instead the seam willburn out where the edges are thus brought together. My process isprimarily one of resistance welding where the welding temperature whichmay be achieved is sufficiently high to ensure complete interfusion ofthe juxtaposed edges but the region where such temperature is devel opedis so limited that the remainder of the tube is only relatively slightlyheated and energy is conserved. Of course, there are various attendantadvantages such as the fact that the squeeze rolls do not becomeoverheated, such rolls are substantially outside the field of theinductor, only a relatively low degree of pressure need be exertedthereby to achieve a perfect weld and. subsequent forging of the same, avery high rate of tube blank travel (and therefore production offinished tubing) is feasible, and a considerable variation in such ratecan be tolerated without corresponding regulation of the power input,squeeze roll pressures, etc. This last is of great importance inasmuchas it permits supervision of the process by a relatively unskilledoperator and ensures a uniform high quality product.

At the point where the seam edges are brought together there may be aslight sputtering due to the break-down of an oxide film, suchsputtering being more pronounced in the case of aluminum than steelsince the insulating effect of aluminum oxide is greater. Similarly, thesputtering is more pronounced in the case of silicon bronze than withcopper. This sputtering helps to clean the opposed seam edges so thatpreliminary cleaning operations are unnecessary. In the case ofmaterials such as steel having a wide plastic range, a forged weld maybe obtained even when insufficient heat is supplied to produce a moltenstate at the point of seam edge juncture. Ordinarily, however, I preferto generate sufficient heat to interfuse the seam edges at this point.

When welding very heavy walled tubing and employing a relatively cycles,it may be desirable to provide a powdered and sintered or bonded ironcore within the tube blank in the region of the inductor to enhance theefficiency of the latter. Such core will be of well-known type and may,mounted on and carried by tube 68. There will ordinarily be no necessityto employ a core, however.

It is generally preferred that the frequency of the current besufiiciently high that effective current penetration will be no morethan about one-thousandths of an inch below the surface of the work. Inthe case of very large diameter tubes, however, where somewhat lowerfrequency ranges may desirably be utilized, this degree of penetrationmay be considerably exceeded.

It will now have been seen that I have provided a method of tube weldingwherein a continuous length of metal strip is formed to general tubularconformation with opposed spaced edges defining a longitudinallyextending open seam, a highfrequency alternating magnetic field isinduced within and around a relatively short longitudinally extendingportion of such preformed tube blank, such blank being continuouslyadvanced through the effective region of such field, such opposed seamedges are brought together at a point spaced a suihcient distance fromsuch region in the direction of travel of such blank that the inducedvoltage across the spaced seam low frequency such as 3000 a.)

for example, be

edges will be substantially lower immediately prior to such point thatin such effective region of such field, thereby causing the inducedcurrent to flow along the opposed seam edges as the combined result ofthe skin effect and proximity effect to such point, where an extremelyhigh current density is obtained, such high current density beingeffective to heat the seam edges to fusion temperature to interfuse thesame, and thereafter such interfused edges are forced further togetherin a forging or upsetting operation.

In practising my invention, the point of welding will ordinarily be adistance beyond the region of high fiux density equal to from about oneto about two diameters of the tube being produced. In the forms ofinductor specifically disclosed above such region of high flux densitymay be considered as terminating quite abruptly closely beyond theextremity of the inductor nearest such point of welding. By point ofwelding or point of juncture as used herein I mean that point at whichthe juxtaposed seam edges are initially interfused, this ordinarilybeing somewhat in advance of the center line of the squeeze rolls. As apractical matter, the immediate subsequent forging step is bothmechanically convenient and advantageous but, of course, may be largelydispensed with if the squeeze rolls are carefully adjusted and the tubeblank perfectly preformed so that the seam edges will be broughttogether into welding engagement very close to the center line of therolls.

My process and apparatus are useful not only in the welding of carbonsteel tubing, but also in a welding tubing of stainless steel, siliconbronze (Everdur), copper, brass, aluminum, and various non-ferrousalloys.

The production of tubing, and particularly thin-walled tubing, inaccordance with my invention is, of course, much less expensive thanconventional tube drawing operations. The metal strip of desired gaugeis very rapidly formed into atubular blank (of round, square, or othercrosssection) and the efiiciency of my method and apparatus permit ahigh rate of production of the welded tubing. By way of illustration,and not limitation, the following examples of actual production runs maybe given:

(a) Low carbon cold rolled steel strip was formed and welded at the rateof 66 feet per minute to produce one inch 0. D. tubing having a wallthickness of .012 inch. To the best of my knowledge no such thin-Walledtubing has ever previously been produced by electric welding methods ofany type. The input to the inductor was approximately 32.8 kw., suchinductor comprising a coil of four turns of copper tubing with T 2- ofan inch spacing between the turns. The inner diameter of the coil was 1%inches.

(0) 3S, half hard, aluminum alloy strip was formed and welded at a rateof 82 feet per minute to produce one inch 0. D. tubing having a wallthickness of .051 inches. To the best of my knowledge, this is manytimes the speed at which it has previously been possible to producewelded aluminum tubing and, in fact, practically all aluminum tubingpreviously commercially available has been of the extruded type.

It should be appreciated that by employment of my new method it is notonly possible to utilize readily available metal strip materials butthat also the highly desirable properties of such materials arepreserved. Thus, cold-working of metal strip ordinarily increases theyield strength, ultimate strength, and hardness of the same but thedegree of such cold ordinary welding procedures tend to produce asubstantial annealed area or region to each side ent from the balance ofthe unaffected remaining portion of the article. In contrast, I amenabled to produce welded tubing, the characteristics of which remainsubstantially uniform about its entire circumference.

When employing my method of welding a vary narrow molten or plasticregion is produced at the weld which may then be further reduced by theforging operation of the squeeze rolls, such oper- (reducing the tubediameter slightly) affords preferential cold working of this narrowweld, since the cast weld is of softer metal than the bodyof the tube,and working of the weld is of course accentuated by the extremenarrowness of a small reduction in tube affords quite considerable coldworking of the weld. The tubing produced my invention therefore not evenin the weld zone.

I have found that when welding aluminum alloy tubing by my new processit is feasible to produce a welded seam in which the width of the castweld region is equal to only about 20% of the tube wall thickness evenprior to my subsequent sizing operation. In prior art methods of weldingaluminum the cast weld portion has always been at least as wide as thematerial was thick, and generally considerably wider.

I have thus been enabled to produce resistancewelded tubing of carbonsteel and stainless steel having a wall thickness of as little as .012inch and even less, and likewise to produce such tubing of aluminum andother non-ferrous metals and their alloys having a wall thickness muchless than ever before possible, with a cast-weld longitudinallyextending scam the width of which is no greater than approximately 20%of the wall thickness. Such tubing may readily be corrugated, forexample, by well-known procedures Without damage thereto at the Weldedseam or weld zone.

The fact that extremely heavy pressure is not required to be exerted onthe blank by the squeeze rolls facilitates the production of suchthinwalled tubing and other pressure means such as a bell may beemployed instead of such rolls although the rolls are preferred.

This application is a division of my co-pending application Serial No.205,511, filed January 11, 1951, for High Frequency Induction TubeWeldmg.

Other modes of applying the principle of the invention may be employed,change being made as regards the details described, provided thefeatures stated in any of the following claims or the equivalent of suchbe employed.

I therefore particularly point out and distinctly claim as my invention:

1. The method of producing continuous welded tubing which comprisescontinuously advancing a metal strip, cold-forming such continuouslyadvancing strip to a blank of desired tubular contour with the edges ofsuch strip opposed but spaced apart to form an open longitudinal seam,generating a high-frequency alternating magnetic field, concentratingsuch field in a circumferentially uniform region relatively narrowlongitudinally of such blank, passing such tubular blank through suchregion, bringing such opposed seam edges together at a point beyond theeffective limits of such region after passage therethrough, such tancebeyond such region in the direction of travel of such blank that theinduced voltage across the spaced seam edges will be substantially lowerimmediately prior to such point than within such region, thereby causinga high-frequency induced current to flow generally circumferentially ofwithin such region and along the opposed seam edges to such point ofjuncture thereof, the combined result of the skin effect and proximityeffect being thus to concentrate such current along such seam edges toan increasing egree toward such point of juncture where an extremelyhigh current density is obtained, such high current density beingeffective to heat the seam edges to fusion temperature the same, andthereafter forcing such interfused edges further together to forge theresultant welded seam.

2. The method of producing continuous welded tubing which comprisescontinuously advancing a metal strip, cold-forming such continuouslyadvancing strip to a blank of desired tubular contour with the edges ofsuch strip opposed but spaced apart to generating a high-frequencyalternating magyond the effective limits or such region after passagetherethrough, such point being spaced a sufiicient distance beyond suchregion in the direction of travel of such blank that the induced voltageacross the substantially lower immediately prior to such point thanwithin such region, thereby causing a high-frequency induced current toflow generally circumfereutially of such blank within such region andalong the opposed seam edges to such point of juncture thereof, thecombined result of the skin effect and proximity effect being thus to isobtained, such high current density being effective to heat the seamedges to fusion temperature and to interfuse the same, and thereafterforcing such interfused edges further together to forge the resultantwelded seam.

3. The method of producing welded tubing which comprises forming agenerally tubular metal blank having an open longitudinally eX- tendingseam, advancing such blank axially, generating a high frequencyalternating magnetic field parallel and concentric with such advancingblank, utilizing such field to generate a more concentrated field.parallel to, concentric with, and partially within such blank and offsetaxially of such first field in the direction of travel of such blank,and bringing the seam edges of such at a point beyond the effectivelimit of such concentrated field in the direction of travel of suchblank, such concentrated magnetic field inducing a voltage in such blankeffecl tive to generate a high frequency current in such blank withinthe region of such concentrated magnetic field with such current flowingfrom such region along the seam edges to such point of meeting of suchedges.

4. In apparatus for the continuous production of welded tubing frommetal strip, including a mill for forming such strip into a tubularblank having an open longtiudinal seam; an insulated seam guide adaptedto enter and position such seam; a high frequency inductor closelyfollowing said seam guide through the field of which such blank isadapted to pass, and squeeze rolls closely following said inductorarranged and adapted to force the opposed edges of such seam together ata point beyond the efiective limit of the field of said inductor butclosely adjacent the same, said inductor comprising a generallycylindrical electrolytic copper shell having a frusto-ccnical taperedend portion protruding toward said squeeze rolls, an inner electrolyticcopper shell secured within said outer shell. and spaced therefromexcept where joined thereto at its ends, said inner shell being ofgeneral conical conformation and having a relatively small diametercylindrical end portion centrally disposed within such frustoconicalportion of said outer shell. axially through which such blank i adaptedto said shell assembly having a narrow radial gap therein, axiallyextending radial partitions connecting said inner and outer shells oneach side of such gap to close the chamber formed between said shells, aconduit communicating with such chamber adjacent one said partition anda second conduit communicating with such chamber adjacent the other saidpartition for circulation of a cooling fluid therethrough, and aninduction coil of electrolytic copper tubing closely encircling thecylindrical portion of said outer shell but electrically insulatedtherefrom, the tubing comprising the turns of said coil being ofrectangular cross-section and such turns being closely spaced but out ofcontact with each other.

5. A high frequenc inductor adapted to generate a uniform fieldsymmetrically of the axis of an elongtaed work-piece such as a tubularblank passed therethrough comprising a generally cylindrical coppershell having a frustoconical end portion, a concentric inner coppershell secured within said outer shell and spaced therefrom except wherejoined thereto at its ends, said inner shell being of generally conicalconformation and having a relatively small diameter cylindrical endportion centrally disposed within such frusto-conical end portion ofsaid outer shell for passage of a work-piece such as a tubular blanktherethrough, said shell assembly having a narrow radial slottherethrough, axially extending radial walls connecting said inner andouter shells on each side of such slot to close the chamber formed bysaid shells, said walls being slightly spaced to provide a narrowradially extending gap therebetween, means adapted to circulate acooling fluid through such chamber, an induction coil of copper tubingclosely encircling the cylindrical portion of said outer shell butelectrically insulated therefrom, said coil extending no further axiallythan the end or said cylindrical outer shell toward said frusto-conicalend portion, the tubing comprising the turns of said coil being ofrectangular cross-section and such turns being closely spaced but out ofcontact with each other, and a plug of insulating material secured inthe large end of said inner shell having a central aperture aligned withsaid small diameter cylindrical end portion,

6. A high frequency inductor adapted to generate a uniform fieldsymmetrically of the axis of an elongated work-piece such as a tubularblank passed therethrough comprising a generally cylindrical outer shellof conductive metal having an axially protruding inwardly tapering endportion, an inner shell of conductive metal concentric within said outershell having a relatively small diameter cylindrical portion centrallydisposed within such tapering end portion or said outer shell forpassage of a work-piece such as a tubular blank therethrough, radiallyaligned slots in said shells, axially extending radial walls connectingsaid inner and outer shells on each side of such slots leaving aradially extending gap therebetween so that an electric current may becaused to flow circumferentially of said outer shell until such gap isreached, then radially inwardly along one said wall, around said innercylindrical portion back to such gap, and then radially outwardly alongsaid other wall to said outer shell again, and an induction coil closelyencircling the cylindrical portion of said outer shell but electricallyinsulated therefrom.

'7. A high frequency inductor comprising a generally cylindrical outershell of conductive metal, a generally cylindrical central inner shellof conductive metal of less diameter and length than said outer shelland protruding axially beyond the end or the latter, each of said shellshaving a longitudinal slot therethrough, longitudinally extending wallsconnecting said inner and outer shells on each side of such slotsleaving a radially extending gap therebetween, whereby an electriccurrent may be caused to flow circumferentially of said outer shelluntil such gap is reached, then inwardly along one said wall, aroundsaid inner cylindrical shell back to such gap, and then outwardly alongsaid other wall to said outer shell again, thus concentrating suchcurrent in a region both centrally of and axially offset from said outershell, and an induction coil closely encircling said outer shell butelectrically insulated therefrom.

8. An inductor unit comprising a generally cylindrical outer shell ofconductive metal, a shorter generally cylindrical concentric inner shellof conductive metal, each of said shells having a longitudinal slottherethrough, longitudinally extending walls connecting said inner andouter shells on each side of such slots leaving a radially extending gaptherebetween, whereby an electric current may be caused to flowcircumferentially of said outer shell until such gap is reached, theninwardly along one said wall, around said inner cylindrical shell backto such gap, and then outwardly along said other wall to said outershell again, thus concentrating such current in a region centrally ofsaid outer shell, said short inner shell being symmetrical to generate auniform field symmetrically of the longitudinal axis of a tubular blanktravelling centrally therethrough, and an induction coil encircling saidouter shell but electrically insulated therefrom.

9. A high frequency inductor comprising a generally cylindrical outershell of conductive metal, a generally cylindrical central inner shellof conductive metal of less diameter and length than said outer shelland protruding axially of the latter, each of said shells having alongitudinal slot therethrough, longitudinally extending wallsconnecting said inner and outer shells on each side of such slotsleaving a radially extending gap therebetween, whereby an electriccurrent may be caused to flow circumferentially of said outer 1 7 shelluntil such gap is reached, then inwardly along one said wall, aroundsaid inner cylindrical shell back to such gap, and then outwardly alongsaid other wall to said outer shell again, thus conexiting therefrom.

10. Seam-welding apparatus for continuously welding the edges of alongitudinally-sp1it tube comprlsing means for advancing said tube alongsaid edges for concentrating current along the edges to an increasingdegree toward said point.

References Cited in the file of this patent Number N umber UNITED STATESPATENTS

